Theoretical diffractive filter performance for ballistic transillumination

We address the topic of selectively probing turbid media, equivalent to biological tissue, with ballistic photons. The samples considered exhibit highly forward-directed scattering (anisotropy factor g > 0.9). We propose the utilization of a non-linear grating-based angular filter to separate the faint ballistic signal from optical noise. The filter is conformed of a monochromatic source incident on a ruled grating, positioned at grazing diffraction, followed by a narrow slit. Minute deviations in propagation angle of scattered radiation are amplified non-linearly. A rotation scheme with 1 arc min resolution may provide angular amplifications in the range of 15X-60X. In addition, similar values of transversal beam size reduction enable efficient ~100X-1000X filtering of optical noise. We utilize random walk Monte Carlo calculations to emulate the interactions between radiation and turbid media. The influence of the filter is considered by employing ray trace analysis and conical diffraction theory of ruled gratings. For samples with g =0.995, the configuration permits a significant ~10e3 reduction of optical noise.